1. Field of the Invention
This invention relates to Fourier filters, systems for fabricating Fourier filters and systems and methods for inspecting a specimen using Fourier filters. Certain embodiments relate to a Fourier filter that includes an array of patterned features formed within an optically opaque layer. A dry process is used to form the array of patterned features by removing select portions of the optically opaque layer to create transmissive regions, which only allow light scattered from the defects to pass through the Fourier filter. Light reflected and diffracted from periodic structures on the specimen is substantially blocked by the array of patterned features.
2. Description of the Related Art
The following descriptions and examples are given as background only.
Fabricating semiconductor devices such as logic and memory devices typically includes processing a substrate using a large number of semiconductor fabrication processes to form various features and multiple levels of the semiconductor devices. For example, lithography is a semiconductor fabrication process that involves transferring a pattern from a reticle to a resist arranged on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing, etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated in an arrangement on a single semiconductor wafer and then separated into individual semiconductor devices.
Inspection processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield in the manufacturing process and thus higher profits. Many different types of inspection tools have been developed for the inspection of semiconductor wafers. Defect inspection is currently performed using techniques such as bright field (BF) imaging, dark field (DF) imaging, and scattering. The type of inspection tool that is used for inspecting semiconductor wafers may be based on, for example, characteristics of the defects of interest and characteristics of the wafers that will be inspected. For example, some inspection tools are designed to inspect unpatterned semiconductor wafers, while others are designed to inspect patterned semiconductor wafers.
Inspection tools for unpatterned wafers are generally not capable of inspecting patterned wafers for a number of reasons. For example, many unpatterned wafer inspection tools are configured such that all of the light collected by a lens or another collector is directed to a single detector. The detector generates a single output signal representative of all of the collected light. Therefore, light scattered from patterned features on the specimen will be combined with other scattered light. This prohibits the other scattered light (e.g., light scattered from defects or other surface irregularities) from being detected separately from the light scattered from the patterned features.
Patterned wafer inspection is of particular interest and importance to the semiconductor industry because processed semiconductor wafers usually have a pattern of features formed thereon. In some cases, unpatterned wafers, or “monitor wafers,” which have been run through a process tool, may be inspected as a gauge for the number and types of defects that may be found on patterned wafers, or “product wafers.” However, the defects detected on monitor wafers do not always accurately reflect the defects that are detected on product wafers after the same process in the same process tool. Inspection of product wafers is, therefore, important to accurately detect defects that may have been formed on the wafer during, or as a result of, processing. In other words, inspecting product wafers may provide more accurate monitoring and control of processes and process tools than inspection of monitor wafers.
Many inspection tools have been developed for patterned wafer inspection. Some patterned wafer inspection tools utilize spatial filters to separate light scattered from patterned features from other scattered light, so that the other scattered light may be separately detected. Since the light scattered from patterned features depends on various characteristics of the patterned features (e.g., lateral dimension and period), the design of the spatial filter also depends on the characteristics of the patterned features. As a result, the spatial filter must be designed based on known or determined characteristics of the patterned features and must vary as different patterned features are inspected.
A Fourier filter is one type of spatial filter that may be used as described above. Fourier filters are relatively useful for filtering light from repetitive patterns, such as memory arrays formed on a wafer, so that defects or other surface irregularities may be more easily detected. Various methods have been used in the past to provide Fourier filtering.
One method of Fourier filtering is the mechanical method. This method utilizes mechanical rods (or other mechanical devices) to block the diffraction pattern generated by array structures, so that the energy from the array is removed from the optical path of the inspection system. Although the mechanical method is widely used, it has a number of disadvantages, particularly for flood illumination based inspection systems.
Flood illumination based inspection systems produce diffraction peaks that show up as “dots” in the image plane. The rods used in the mechanical method block excessive amounts of light in these systems, thereby reducing the overall defect signal. In addition, the mechanical method induces significantly more Fourier filter ringing, which reduces defect sensitivity. For example, Fourier filters in the form of periodic blocking bars diffract light into undesirable directions, inducing “ringing” or side lobes in the defect signals. In some cases, the side lobes may produce significant distortion at the image plane of the inspection system, adversely affecting the ability of the inspection system to detect defects on the wafer with high accuracy. Furthermore, the rods must have a relatively large diameter in order to be structurally sound. As such, only a limited number of rods can be used; otherwise, the entire plane would be blocked by the rods.
Another method for Fourier filtering is the liquid crystal method. This method utilizes a one- or two-dimensional liquid crystal device to block the diffraction pattern generated by array structures, so that the energy from the array is removed from the optical path of the inspection system. Currently used liquid crystal Fourier filter devices are programmable and capable of filtering out diffraction dots in a two-dimensional manner. However, there are several factors that make liquid crystal devices inappropriate for flood illumination based inspection systems. For example, the use of a liquid crystal device as a Fourier filter utilizes the principle of light scattering. However, light scattering significantly alters the wavefront of the inspection system, causing severe degradation to the image quality. In addition, most liquid crystal material has a damage threshold at a wavelength of approximately 300 nm, making liquid crystal devices less than ideal for use in deep ultraviolet (DUV) based systems (i.e., systems that operate at wavelength(s) less than about 300 nm).
Chrome masks have also been used for Fourier filtering. Chrome mask Fourier filters are generally fabricated with optical or electron beam lithography processes to include opaque chrome regions arranged in a pattern on a substantially transparent substrate. Masks fabricated with such processes tend to provide good repeatability and matching and have been used to make ICs for many years. For example, a chrome mask may be fabricated by depositing chrome on the substrate, patterning the chrome by lithography and etch, and cleaning the substrate with the patterned chrome formed thereon. As such, a chrome mask Fourier filter may be tailored or “programmed” to block the diffraction pattern produced by a particular array of structures.
However, chrome masks are not used in many inspection systems due to the relatively long turn around time and relatively high cost associated with fabricating such filters. For example, most semiconductor fabrication facilities do not have dedicated lithography and cleaning stations for Fourier filter fabrication. As such, chrome mask designs are often sent out to a mask shop for fabrication. However, sending filter designs out for fabrication often results in several days of down time for the inspection system. This makes chrome mask Fourier filters unsuitable for situations in which a fast turn around time is beneficial (such as during process development).
Accordingly, a need exists for an improved Fourier filter, which overcomes the disadvantages described above. In particular, a need exists for a two-dimensional, programmable Fourier filter suitable for flood illuminated dark field wafer inspection systems, which: (i) does not block excessive amounts of light, (ii) does not induce significant Fourier filter ringing or periphery energy leakage, (iii) does not significantly alter the wavefront of the inspection system, (iv) does not cause degradation of the image, and (v) does not suffer from long turn around time or high cost. The Fourier filter should also be structurally sound and suitable for use at DUV and other wavelengths.